So I'm 19 years old and I was a virgin until a few weeks ago when me and this girl I'm friends with randomly tried to sleep together. She's really experienced in the sex department, although she hasn't slept with anyone in a while, whereas I'm obviously not. We have a complicated relationship in the sense that we both really like each other, but for some reason she doesn't want to be with me (plus, aside from that, we're best friends). Anyways, so we started by making out, and than she guided my hand down toward her vagina and we had that whole thing going for a while. Than all of a sudden she asked if I wanted to have sex, and I said yes. I didn't have an erection after making out or touching her vagina, and she said this was really surprising. I had to masturbate myself to get one, and she couldn't give me one with a handjob. Anyways so once I had an erection, I put the condom on and she got on top of me, and after about a minute it went away. I couldn't really get it back and she just decided she was tired and went to bed. She said she felt it though. And it made me worry a lot but i figured it might just be a fluke. Anyways, so than today, about 2 weeks later, she wants to try having sex again. So of course I agree. So we drive to a backroad..and i've been waiting for this for 2 weeks. but the same thing happens again! when i take my pants off i dont have an erection and shes really surprised. so i eventually masturbate and get myself one again. She tried to give me an erection through a handjob and oral but it just didnt work, so i had to masturbate myself. So eventually i get it going again and after she gets on top of me and we're having sex, I lose it again...it really sucks, and i know shes probably thinking what the hell over there. I know she was really mad at me because i told her i couldnt feel anything...which is true..i couldnt really feel any sensation except when im doing it myself. I dont feel any sensation in my penis during sex, so im wondering what the heck is going on. Its like i cant just do it because im worried about it for some reason. anyways, I'm in decent shape, and I have no trouble masturbating at home when i'm alone..im just so worried that i wont be able to do it with her and she'll eventually want to stop trying..i like her a lot and i dont want to lose her..and she already wont be with me but at least shes trying to have sex with me..i probably masturbate about once a day. I think she's not too bad looking so thats not the issue. i really want to do this with her too. I don't know what the problem is. Im thinking it could be:
1) I'm really nervous...(i dont know how not to be?)
2) I'm just used to touching myself, and i like how that feels.. (so maybe thats why i dont feel a sensation here)
3) i dont even know? what the heck!

I'm thinking of possible solutions and I'll probably consult a doctor I'm so worried about it...i don't know I just need some help and advice on what to do. im soooooo worried about all of this. i need to finish with her. please help!